Described below is a conventional front-end integrated circuit in which an RF (Radio Frequency) circuit and a digital demodulating circuit are integrated in one package.
A one-packaged front-end integrated circuit 100, which is generally used, is provided with an RF circuit section 110 and a digital demodulating circuit section 120 as shown in FIG. 8. The RF circuit section 110 and the digital demodulating circuit section 120 are in one package.
The RF circuit section 110 is provided with an RF variable gain amplifier 101, a modulating section 102, and a base band variable gain amplifier 103. The RF variable gain amplifier 101 amplifies a high frequency signal. The modulating section 102 performs quadrature modulation of the high frequency signal into an. I/Q base band signal. The base band variable gain amplifier 103 amplifies the I/Q (Inphase/Quadrature) base band signal.
Moreover, the digital demodulating circuit section 120 is provided with an analog-to-digital conversion circuit 121 for converting the I/Q base band signal into an I/Q digital signal. Thus, the digital demodulating circuit section 120 demodulates the I/Q digital signal.
In such front-end integrated circuit 100 in which the RF circuit section 110 and the digital demodulating circuit section 120 are integrated in one package, the RF circuit section 110 and the digital demodulating circuit section 120 are internally connected via base band signal (BBS) lines 131 and an amplification ratio control signal line 132 (hereinafter, an amplification ratio control signal is referred to as an automatic gain control (AGC) signal).
In normal operation, a feedback loop is constituted so that an input signal level of the digital demodulating circuit section 120 is kept constant (an input signal to be inputted into the digital demodulating circuit section 120 has a certain level constantly). In short, a base band signal (BBS), which is an I/Q base band analog output from the RF circuit section 110, is supplied to the digital demodulating circuit section 120. Then, analog-to-digital conversion of the base band signal is performed. Here, an amplification ratio control circuit 122 of the digital demodulating circuit section 120 is constituted of a digital circuit. The amplification ratio control circuit 122 detects a level of a base band output that has been digitalized. Then, in accordance with the level thus detected, the amplification ratio control circuit 122 outputs a digital AGC signal so that the input signal to be inputted into the digital demodulating circuit section 120 has a certain level constantly. The digital AGC signal is converted from digital to analog. Then, the AGC signal thus converted is supplied to an AGC input terminal of the RF circuit section 110.
As to the RF circuit section 110, the following properties may be tested, for example,                Gain property, such as maximum gain, minimum gain, a maximum variable range of gain, and the like,        Level differences of I/Q base band signal (BBS),        Phase differences (quadrature property) of I/Q base band signal (BBS), and        Phase noise property.        
An example of a conventional testing method as to the RF circuit section 110 of the one-packaged front-end integrating circuit 100 is a test disclosed in Japanese Publication of Unexamined Patent Application “Tokukai No. 2002-232498 (published on Aug. 16, 2002). In this testing method, a test is carried out while an automatic gain control loop (hereinafter, referred to as an “AGC loop”) is closed.
However, in the conventional integrating circuit, a terminal from which an output signal of the digital demodulating circuit section 120 is outputted is directly connected to the AGC input terminal that is connected to the RF variable gain amplifier 101 located inside the RF circuit section 110, thereby forming the AGC loop. It is a problem that it is impossible to test, in a short time, the amplification ratio property and the like of the RF variable gain amplifier 101 included in the RF circuit section 110.
That is, for the test to find values of the maximum gain and the minimum gain, or the maximum variable range of gain from the values of the maximum gain and the minimum gain, it is necessary to sweep the level of the input signal and evaluate an AGC signal level after convergence of the AGC loop. Alternatively, it is necessary to monitor the BER before error correction in order to find a level at which a bit error ratio (BER) becomes less than a certain value. Such test needs a long time.
Similarly, as for tests that do not depend on the output level, such as a test of phase noise by using a voltage controlled oscillator (VCO), a test of I/Q phase difference, and the like, it is necessary to program the tests such that a waiting time for waiting the convergence of the AGC signal is included. Thus, a test time of such tests is long in vein.
Moreover, even if the control is possible, the base band signal is not outputted to outside of the integrated circuit, lest the BER be reduced. This constrains the property tests (tests on the properties) of the RF section only. Especially, monitoring a BER property of a reception signal is the only way to evaluate the properties, such as the I/Q phase difference and phase noise, and needs a long time.
Furthermore, the test of the RF circuit section 110 requires that the digital demodulation circuit section 120 be provided with means for testing the RF circuit section 110. This leads to inaccuracy of the test and scale up a size of the circuit.
Even if an output terminal is provided, a driving circuit should be provided outside the integrated circuit in order to connect the output terminal with a measuring apparatus. Thus, when a test is carried out by using an I/Q signal, a result of the test is under influence of relativity of the driving circuit thus externally provided. Thus, a circuit using a discrete circuit need be designed with much care.